This invention relates generally to the field of precision movement control and, more particularly, to the use of ultrasonic motors for movement control.
The support and driving of moving objects forms the subject of a very large body of technical art. As the field has matured, the focus of advancements has gradually shifted to such beneficial system attributes as smoothness of motion, suppression of vibration, longevity of moving parts, tolerance of manufacturing imperfections, low weight and low cost. Of these, the first three attributes (smooth motion, vibration suppression and moving part longevity) tend to be achieved at the detriment of the last three (manufacturing imperfections, weight and cost). For example, longevity often is associated with a smooth, vibration free motion such as can be obtained using precision bearings and other expensive components. Conversely, the use of low cost or light weight parts may lead to shortened operating life and a rough ride that may be incompatible with sensitive payloads.
The elements of a motorized system essential to the present invention are the following:
1) Stator or support
2) Motor or actuator
3) Rotor or driven member
4) Coupling means between motor and stator (e.g. fasteners or springs)
5) Coupling means between rotor and motor (e.g. gears or friction tips)
6) Optional coupling means between rotor and stator (e.g. bearings)
Coupling among these various elements often involves some form of preload, particularly when manufacturing imperfections are expected and when the avoidance of backlash or loss of control is desired. In the case of a friction drive system, a preload between motor and rotor is inherently required in order to achieve the desired frictional coupling. The preload frequently affects the stresses, and the effectiveness and durability of the bearings supporting the rotor. In its turn, design and manufacturing imperfections in the bearings affect the quality of the movement. The same preload can also induce deformation in both moving and stationary structures. The deformation can be elastic (temporary), plastic (permanent), or some combination of elastic and plastic. In any of those cases, such deformation is accompanied by dynamic loads that are frequency dependent, leading to mechanical vibration, acoustic noise, limitations of operating speed and premature failure. These effects are, of course, not limited to a friction drive, but can be found in any coupling application where the motor preload is transmitted through a compliant structure to a bearing.
U.S. Pat. No. 7,199,507 issued in the names of Ganor et al., and particularly FIG. 1A of this patent, 1 illustrates this well known prior art configuration in the case of a friction drive where the preload is applied radially. The rotor of this example is configured as a relatively thin shell. Although such a thin shell saves material and cost, its slenderness subjects it to the risk of deformation, increased vibration and a shortened life. Other prior art examples might opt for a thicker rotor, or even a solid rotor, in order to avoid such undesirable side effects. The effects discussed above also apply if the preload is applied in an axial direction, as seen in the prior art example of U.S. Pat. No. 5,682,076 issued in the name of Zumeris, and particularly FIG. 14C of that patent.
A number of approaches to mitigate such unwanted side effects are commonly found in the art, as summarized in Table 1:
TABLE 1Mitigation approachTradeoff/penaltyOperation under reduced loadReduced performanceOperation at lower speedReduced performanceUse of stiffer componentsHigher cost/weightUse of higher quality componentsHigher cost
As can be seen in Table 1, the mitigation approaches most commonly found carried with them significant penalties. More recently, additional approaches have been introduced to improve unintended tradeoffs (Table 2).
TABLE 2AdditionalmitigationapproachesTradeoff/penaltySymmetrical motorAdded motor costarrangementIncreased driving complexityrelieves load onBearing and motor contact over-constrain the rotor:rotor bearingscompliance/preload still needed to accommodate(As in U.S. Pat.part and assembly imperfectionsNo. 5,714,833)Performance limited by driven member surfaceaccuracySymmetrical motorAdded motor costarrangementIncreased driving complexitysupports rotorDiminished load carrying capacitywithout bearingsCompliant rotor mounted in deformed state limits(As in Pat. No.speed and operating life7,687,973)Performance limited by driven member surfaceaccuracySymmetricalCompliant rotor mounted in deformed state limitsarrangement ofspeed and operating lifemotors and idlersOne sided driving results in center of massprovides a lowermovement, reduced resonant frequency and morecost method ofvibrationrelieving load onPerformance limited by driven member surfacemoving memberaccuracysupports (As in Pat.Nos. 6,617,759 and5,424,597)Counter rollerComplex, two-active element motor used to securecarried by motorconsistent relationship of motor to relatively movingassembly removespartmotor preload fromMotor complexityload presented byKinematic load variations due to non-ideal shape ormoving membersurface conditions not provided for(As in Pat. No.Payload supported by weak contact points5,682,076)
For instance, a symmetric arrangement of preloaded motors, as shown in FIG. 31B of U.S. Pat. No. 5,714,833, could substantially remove the portion of the bearing stresses due to the motor. Moreover, in the case of a light payload such as a hard drive read head, the bearing could be eliminated entirely by having the rotor completely supported by the motors themselves, as taught by U.S. Pat. No. 7,687,973, and particularly FIG. 6A of that patent. Symmetrically pressing against a driven member was broadened to include both active motors and passive, or idling, rollers, as taught in U.S. Pat. No. 6,617,759, and particularly FIG. 3 of that patent. A similar approach is taught in U.S. Pat. No. 5,424,597 issued in the names of Gloss et al.
As also disclosed by Zumeris in the aforementioned U.S. Pat. No. 5,682,076, and particularly in FIG. 21 of that patent, a counter-roller could be introduced, which would balance the preload locally. Mounting the counter roller onto the same structure as the motor was a significant advancement since it allowed maintenance of the preload value throughout the available range of motion without risk of denting the engaged surface, at least for the case of the motor assembly being the travelling element. In spite of that advancement, certain limitations have persisted. According to the '076 patent specification, the embodiment of its FIG. 21 could be operated with either the motor or the engaged member being the travelling element. In the figure, it is the motor with its housing that are allowed to travel. Although this arrangement is clearly possible, it is not very advantageous for the following reasons:
In normal operation, the payload and the motor are supported by contacts between the motor and a relatively thin engaged body, on the one hand, and between ceramic spacers and a counter roller/bearing on the other hand. The amount of payload that can be carried is thus limited by the mechanical strength of the piezoelectric ceramic, a material known to be relatively brittle.
In addition, since the motor requires a source of power, its travel range is limited by the length of wire, cable or flexible circuitry that can be carried by the moving motor. Alternatively, if a battery is chosen as the source of power, the payload capacity must be reduced by the mass of the battery and associated electronics. This limitation, of course, is shared by any “locomotive” type of transport.
Another mode of operation of the invention disclosed in U.S. Pat. No. 5,682,0767 envisions the motor to be stationary. In this mode certain limitations remain, as well: First, as in the other mode, the payload is supported by the contacts between the engaged body, on the one hand, and the ceramic spacers and counter roller/bearing on the other hand. The amount of payload that can be carried is thus limited by the mechanical strength of the piezoelectric ceramic, a material known to be relatively brittle. No other means of supporting the payload is provided. Second, as the driven member advances through its range of motion, a variable torque would accompany the change in payload moment arm length. In turn the mechanical load presented to the two piezoelectric elements would vary in an imbalanced way, thus further reducing the payload capacity.
A further limitation, which applies to either of its modes of operation, is that constructing the motor with two active piezoelectric vibrators essentially doubles the complexity and cost, as well as the probability of failure of the motor.
Although the approaches of the prior art described above have enabled remarkable performance improvements, the widespread growth of applications with demanding movement specifications continues to be hampered by the cost of high precision driven members such as ceramic rotors and sliders. Accordingly, it is a general object of the present invention to overcome the aforementioned shortcomings and limitations while retaining the known benefits of the existing art. It is a particular object of the invention to provide an apparatus for coupling between a motor and a driven member, said apparatus which maximizes positive system attributes such as motion smoothness and tolerance to component imperfections, while minimizing negative attributes such as high cost, weight and sensitivity to the deterioration of components under stress. It is a further particular object of the invention to provide a method of coupling a motor and a reduced-cost driven member, such that the effects of bearing loads, system weight, sensitivity to component imperfections and cost are minimized while stiffness in the direction receiving the motive force is maximized. Yet another object of this invention is to provide a low cost, high performance, rotation stage.
It will be appreciated from the foregoing that there is still a significant need for a method and related apparatus for precisely controlling the movements of a driven member, which may have imperfections arising from manufacture, wear, or simply poor quality, with a drive motor and an associated coupling mechanism, in such a way as to achieve a high level of positioning performance without the burdens of increased cost and weight normally associated with that level of performance. The present invention satisfies this need and has other aspects advantages that will be discussed in the following summary.